This invention relates to a radio receiver and particularly although not exclusively to a radio pager using a code format to the POCSAG standard, which is a British Post Office Standard for digital data transmission.
Our British Pat. No. 1517121 describes a radio pager for FSK signals and similar to the STC Model 5C pager. This pager is a tone-only receiver operating in the VHF high band (138-174 MHz). It uses a binary digital code format to the POCSAG standard, which provides the receiver with four addresses, each distinguished by a different alert tone pattern.
In order to make it possible to produce the complete radio receiver on a single chip, a direct conversion technique is employed in which the radio frequency signal is mixed with a local oscillator on the nominal channel frequency, resulting in a "Zero I.F.". Because the radio signal is frequency modulated there is in fact a beat frequency produced in the mixer, equal to the frequency deviation of the carrier. Conventional I.F. strips are replaced by audio frequency amplifiers with this technique, and adjacent channel selectivity is provided by low pass filters.
To make it possible to recover frequency modulation, the radio frequency signal is fed to two paths with a 90 degree phase difference between them. Each path has its own mixer, fed from the same local oscillator and its own low pass filter and limiting amplifier. The phase relationship of the two baseband signals will vary according to whether the radio carrier frequency is above or below the local oscillator frequency. The two baseband signals are combined to recover the modulating signal in a D-type clocked flip-flop to the D-input of which is applied the output of one of the limiting amplifier stages whilst the output of the other limiting amplifier stage is applied to the clock input of the flip-flop.
In the POCSAG radio-paging network, the binary data is modulated onto the r.f. carrier using F.S.K. modulation, as discussed above. The data rate is 512 Bits/sec and a frequency deviation of .+-.4.5 KHz is used. This high modulation index produces large concentrations of energy separated .+-.4.5 KHz away from the r.f. carrier. The baseband (low pass) filters that are used in the homodyne (zero I.F.) radio-paging receiver only need to be bandpass filters centered on 4.5 KHz and having a noise equivalent bandwidth of around 1 KHz to produce optimum receiver detection performance. However this argument assumes that the receiver's local oscillator is exactly on frequency or with a very small frequency offset (about 100 Hz). This cannot be achieved with a small, cheap crystal oscillator directly.
A compromise solution adopted on the present Model 5C pager manufactured by Standard Telephones and Cables plc is to use a much larger noise equivalent bandwidth (6.5 KHz) to allow for crystal oscillator temperature drift and crystal aging. The filters then become band pass filters with a 6.5 KHz upper cut-off frequency, the lower cut-off being around 1 KHz, with associated loss of receiver sensitivity.